In recent years, the use of floating facilities has become prevalent in the offshore oil and gas industry for the extraction and production of oil and gas from offshore fields. Floating facilities such as the floating, production, storage and offloading (FPSO) unit or the floating, storage and offloading (FSO) unit are typically floating platforms converted from oil tankers or in some cases vessels or barges specifically designed and built for the purpose.
The growth in interest of such floating facilities is due to the ease of installation, the minimal fixed infrastructure required, flexibility in usage and most importantly the ability to weathervane in response to the prevailing weather conditions as compared to the fixed oil platforms or oil rigs. When these floating facilities are moored in the open sea and exposed to prevailing weather conditions, their ability to weathervane freely stems from the mooring system that they utilize. One such mooring system is the turret mooring system.
WO2009/031971 describes a turret mooring system where a turret in the form of a vertical cylindrical shaft is fitted through a moonpool within the floating platform. The flange on one end of the cylindrical shaft is in turn mounted on a turret bearing system which is fixed to the deck of the vessel. On the other end of the cylindrical shaft, a plurality of catenary mooring lines extends out from the shaft to anchor it to the seabed. In addition, a plurality of production risers runs from the wellheads on the seabed to the same end of the cylindrical shaft. The oil is then transferred from the topside of the shaft to the oil processing equipment or storage tanks of the floating platform. This assembly of the turret, the mooring lines, the turret bearing system and the deck of the vessel allows the turret to be fixed to a position with respect to the seabed and the floating platform to rotate or swivel about the vertical axis of the cylindrical turret, enabling the weathervane ability of the floating platform.
In this turret mooring system, the turret bearing system consists of roller assemblies arranged along the circumference of the circular turret bearing system with the axes of the cylindrical rollers pointing toward the axis of the circular bearing. The roller assembly 100, as shown in FIG. 1, consists of a single rotatable cylindrical roller 102 supported by a non-rotating axle 104 which is bolted to the hydraulic supports 106, 108 on each end and in turn are fixed to a base plate 110.
The hydraulic supports 106, 108 form a hydraulic system whereby they are interconnected via a hydraulic line sharing the same source of hydraulic fluid. In such a set up of the hydraulic system, the two hydraulic supports 106, 108 would act in a counteracting manner. Therefore when a downward force acts on one of the hydraulic supports e.g. 106 and causes it to be depressed, the hydraulic fluid would be forced to flow to the other hydraulic support e.g. 108 and causes it to be elevated.
In the normal operation of the floating platform out at sea, the floating platform is susceptible to environmental conditions and would naturally roll or pitch following the prevailing weather conditions. Such motion would cause radial loading on the turret bearing system. The turret bearing system 200 described in WO2009/031971 attempts to cushion the radial movement of the shaft by interspersing radial bearing assemblies 202 with the roller assemblies 100 as shown in FIG. 2. However, due to practical considerations, including the difficulties in positioning the turret entirely centrally within the moonpool, the radial bearing assemblies 202 still have to allow some movement of the shaft. Thus, the radial loading on the roller assemblies remains an issue.
It would therefore be an improvement in the art if a roller assembly and/or turret bearing system could be devised which overcome one or more of these deficiencies.